This invention relates to phonograph pickup transducers, and more particularly to the stylus assembly of such a transducer.
A phonograph pickup transducer senses sound which has been recorded as undulations in a groove cut into a recording surface. Today, flat disc records are almost universally used for entertainment purposes, although the surface of a drum, a flexible belt, and other surfaces can be and have been used.
As the grooved surface is moved with respect to the transducer, the undulations in the groove set the stylus into vibration and these vibrations are desirably an accurate reproduction of the original acoustic vibrations of the sounds recorded. The mechanical stylus vibrations are then converted into an electrical signal having a voltage which varies with the velocity of the mechanical vibrations. The resulting electrical signals can be amplified to drive a loudspeaker, modulate a radio signal, and so on.
A variety of devices may be used to convert the mechanical vibrations of the stylus into electrical signal variations: the stylus may vibrate a moving magnet to induce a time-varying voltage in a stationary coil; or vibrate a moving coil or armature positioned in a stationary magnetic field; or apply vibrating force to induce voltage fluctuations across a piezoelectric crystal or a ceramic element.
That portion of the mechanical-to-electrical transducer which is vibrated by the stylus will hereinafter be referred to as the "armature". The stylus, armature, and other vibrating portions of the transducer are here collectively referred to as the "moving system".
In all of the phonograph pickup systems noted above, the moving system must be supported and properly oriented, and the supporting arrangement which is used is crucial to the fidelity of the overall reproduction system.
First, the moving system and its support must provide high trackability.
"Trackability" indicates how well the stylus tip can follow the undulations in the record groove without losing contact with the groove's surfaces. To extract the recorded information from these undulations, continuous contact with both groove walls is essential. Loss of contact, called "mistracking", produces distortion which is particularly objectionable because, unlike other forms of distortion which may occur within the pickup, electrical signal shaping networks cannot compensate for the information lost.
Trackability can be improved by increasing the "stylus force" which urges the stylus tip against the recording surface or groove walls. Unfortunately, increased stylus force causes increased frictional wear, both on the stylus tip (which can be replaced) and on the record (which often cannot be replaced). Therefore, in order to limit damage to the record under prolonged use to acceptable levels, a low "stylus force", typically on the order of one gram, is desirable. The stylus support arrangement used should accordingly allow the stylus force to be easily adjusted to the desired level, and then maintained at that level.
To maintain trackability with a low stylus force, the rotational inertial mass of the moving system must be kept low. Thus, by reducing the size of the moving system, lower tracking forces may be used without decreasing trackability. Although physical and electrical considerations limit the extent to which the moving system can be miniaturized, the preferred moving system is quite small, complicating fabrication and assembly. It is therefore desirable that the arrangement used to support the stylus and armature be composed of a small number of simple, uncomplicated parts which may be easily, but precisely, assembled. Moreover, because the application and control of adhesives on tiny surfaces is very difficult, limiting the need for adhesives is most desirable.
The low rotational inertial mass of the moving system about its pivot can also be reduced through careful design. For example, a stereophonic moving magnet phonograph pickup having significant advantages is described in U.S. Pat. No. 3,077,521 which issued to E. Ahrens et al. In that arrangement, the stylus is attached to, and extends outwardly from, a permanent magnet. The stylus and armature are supported by an elastomeric bearing which surrounds the magnet positioning it for pivotal motion within a socket formed by the spaced-apart pole pieces of a magnetic coil.
As the frequency of vibration increases, however, the behavior of the moving system becomes much more complex. The moving system is itself a mechanical system that has resonances and these resonances can cause unacceptable distortion if not suppressed or controlled. Although the use of an elastomeric bearing to support the moving system does provide some vibrational damping, additional selective supression of unwanted vibrations at the resonant frequencies of the moving system is desirable.
Techniques for selectively damping the moving system of a transducer have been tried in the past. One such technique is described in U.S. Pat. No. 3,954,273 (Shaper et al--May 4, 1976). The techniques claimed in the Shaper et al patent are identical to those earlier explained in U.S. Pat. Nos. 1,996,104 and 2,031,948.
The present inventors employed the techniques described in U.S. Pat. Nos. 1,996,104 and 2,031,948 in a transducer sold more than one year ago in the United States. The transducer was like the one described in the drawings of this specification, except that member 50 was molded entirely from ENJAY Butyl HT. Although member 50 improved the damping characteristics of the moving system, the inventors have discovered that overall performance can be markedly increased by fabricating member 50 from two different materials, while retaining its one-piece molded construction.